Looks like one of the cow parsley related plants. Perhaps not something you want as it will choke out the rest of your wild flowers.

Thanks Urbanite . . . yes it could be related to cow parsley as there's plenty of that in the wild patch. I've added a third photo to show the difference in the leaves: this new one is certainly a giant.

Angelica also crossed my mind until you posted a picture of the leaves.
Take a look at http://www.wildflowers-and-weeds.com/Plant_Families/Apiaceae.htm to see if anything jumps out at you!

Thanks again Urbanite . . . a great website. So many of the plants look like mine, but I'm opting for Daucus carrota as I did sow some seed there last year, but expected it to be smaller and more delicate.

Your very own Jurassic World!